This invention relates to a selected group of N-(oxazolidinothio)imides active as scorch retarders both in unsaturated elastomers (e.g. SER, natural rubber, Neoprene) and in elastomers with a very low degree of unsaturation (e.g. EPDM). They are active with ultra accelerators, do not cause blooming, are thermally stable, and are not detrimental to the aging properties of the vulcanizates. The selection of these compounds is highly specific and is limited by the nature of both the cyclic amine moiety and the imide.
Blooming is objectionable in rubber items for a variety of reasons, such as:
(a) technical [adhesion of rubber layers to each other or to other substrates is weakened or even prevented (for the effect of bloom on tack see O. K. F. Busenmaker, Rubber Chemistry and Technology, 37, 1178 (1964); J. A. Schaldeman, "Novel Theory for the Action of Phenolic Tackifiers" Paper No. 13, Rubber Div. of A.C.S., 112th Meeting of Oct. 1977); defects in the function of electrical parts insulated with rubber or made of rubber]; PA1 (b) health [possible skin irritation and/or sensitization]; PA1 (c) economic [off-spec items; lower quality items]; and PA1 (d) aesthetic [surface of rubber item is dull or dusty, or oily, slippery or sticky].
Considerable concern over blooming exists in the art. For example, see U.S. Pat. No. 3,780,001.
The very limited class and highly selective retarders of this invention do not cause blooming in non-blooming rubber acceleration systems, contrary to N-(alkylthio)imides, such as N-(cyclohexylthio)phthalimide that is a widely used retarder (see U.S. Pat. No. 3,546,185); N-(aminothio)imides outside of this class, such as N-(morpholinothio)phthalimide, N-(piperidinothio) phthalimide, and N-[(N',N'-benzylethylamino)thio]phthalimide produce blooming in such systems. These blooming imides are described in U.S. Pat. Nos. 3,838,114 and 3,928,340. Certain oxazolidinothioamides, such as N-phenyl-N-[(2-trichloromethyloxazolidino)thio]benzamide and N,N'Bis[(2-trichloromethyloxazolidino)thio]oxanilide, described in this application for comparison purposes and related to known aminothioamides such as are disclosed in U.S. Pat. No. 3,910,864, likewise are undesirable because of excessive blooming.
Thus the non-blooming characteristics and, as shown later, even the scorch retarding activity of the oxazolidinothioimides are not predictable within their class of compounds and are limited to the relatively small selection of structures. The new retarders do not cause blooming even when used at levels higher than necessary. Such excess use of retarder can be highly advantageous as the retardation effect is increased which can overcome or prevent poor results from imprecision such as weighing errors with respect to non-blooming rubber processing ingredients.
Additional examples illustrative of lack of predictability in blooming (and in scorch retardation) are: (a) according to U.S. Pat. No. 3,780,001, the retarder N,N'-bis(cyclohexylthio)oxanilide blooms less while Santoguard PVI, i.e. N-(cyclohexylthio)phthalimide, (hereinafter "PVI", causes more bloom at higher than at lower levels; (b) the oxalamide N,N'-bis(cyclohexylthio)oxamide does not bloom in EPDM but its diphenyl-derivative N,N'-bis(cyclohexylthio)oxanilide causes blooming both in EPDM and in natural rubber at the practical level of 0.5 phr. (parts per 100 parts elastomer or rubber); (c) some oxazolidino-derivatives of amides and imides, including oxalamides either bloom or do not have any significant scorch retarding activity as shown in Tables 1 and 2, infra for N-[spiro(cyclohexane-1,2'-oxazolidino)thio]phthalimide, N-[spiro(cyclohexane-1,2'-oxazolidino)thio]succinimide, N-phenyl-N-[(2-trichloromethyloxazolidino)thio]benzamide, N,N'bis[(2-trichloromethyloxazolidino)thio]oxanilide, N-butyl-N[(2-trichloromethyloxazolidino)thio]acetamide, N-[(2-trichloromethyloxazolidino)thio]diacetimide and N,N-bis[(2-trichloromethyloxazolidino)thio]acetamide.